This invention generally relates to a system and method of storing and controlling the flow of noble gas propellants. This invention specifically relates to a system and process for storing and controlling the flow of noble gas propellants for an ion propulsion system.
Ion propulsion systems are projected to play an increasingly important role for future planetary exploration, and for commercial and military earth orbit satellites. These systems produce larger specific impulses than their chemical thruster counterparts, yielding more efficient, lighter-weight propulsion systems.
Currently, noble gases (e.g., argon, krypton, xenon) are favored propellants for electric or ion propulsion systems, in part, because they are non-corrosive and generally inert. Xenon is popular for such systems because it features (i) a low ionization potential and (ii) a large molecular weight of 131.3 g/mole. These compounds, located in Group VIIIA of the Periodic Table of the Elements, are sometimes referred to as "noble gases" even though they may exist as gas, liquid, or solid.
In present ion propulsion systems, a controlled flow of noble gas (e.g., argon, krypton, xenon) in a gaseous state is delivered at low pressure to a thruster from a compressed gas ("CG") storage vessel. The controlled flow of gas at low pressure (e.g., 20 pounds per square inch gauge ("psig")) from storage at high pressure (e.g., 1200-3000 pounds per square inch absolute ("psia")) is accomplished by a combination of pressure and flow control devices. The volumetric flow rate is generally maintained at between about 2-50 standard cubic centimeters per minute ("sccm") for current satellites. The corresponding mass flow rate for xenon would be approximately 0.2-5 milligrams per second ("mg/s"). Larger flow rates may be required for larger spacecraft or for planetary exploration.
The pressure and flow rate of a noble gas propellant must be controlled in an ion propulsion system to provide stable and repeatable delivery to a thruster. The pressure and flow are typically controlled using combinations of pneumatic valves, solenoid or restrictor valves, expansion volumes, dual solenoid "bang-bang" assemblies, flow restrictors, and heated flow restrictors and plenums. These currently practiced means of regulating pressure and flow can be expensive, are unreliable in the presence of two-phase ("2-P") fluids, are complicated, and can be a source of mechanical system failure since most are comprised of mechanically moving parts. These devices may also consume considerable power. For example, a heated flow restrictor can also consume 100-140 watts of power and some valves must be energized when not in use or on stand-by, which is a majority of the time in current satellites.
Compressed gas storage systems face inherent engineering challenges. First, they simultaneously must maintain high storage pressures while providing a regulated release of gas at lower pressures (e.g., 20 psig to vacuum). Second, as the gas is used and the pressure of the vessel decreases, additional hardware and/or controls must be used to maintain a consistent flow rate. Third, plenum volumes must sometimes be used to store compressed gas at desired pressures to be used by the thruster component, and this hardware may or may not be redundant for multiple or redundant thrusters. For the case in which these plenum volumes are used by more than one thruster, the one or more thrusters are forced to operate at similar thruster feed flow rates. Fourth, the noble gas used in ion propulsion systems are required to be of very high purity. Impurities in the noble gases fed to the thruster can cause erosion, wear or other operational problems in the thruster component.
Another difficulty with compressed gas storage systems arises in space-based applications where the noble gas exists in multiple phases. This is problematic because gas-liquid two-phase systems are difficult to separate in zero-gravity, necessitating separation of the phases, storage tank and plenum heaters, and/or a pre-expansion of the two-phase mixture in order to ensure gaseous delivery to the flow and pressure control devices.
For example, when exposed to the cold temperatures of outer space, compressed xenon gas condenses to a liquid phase resulting in a two-phase system. Xenon has a critical temperature of 290 K which falls in the temperature range experienced by gas storage systems on orbiting satellites, e.g., 90 K to 450 K. The exact range depends, in part, on the insulation and temperature controls employed. If the temperature drops below 290 K in space, the potential exists for coexisting liquid and vapor phases. To mitigate this problem, heaters would have to be used and the location and configuration for such heaters is not trivial in zero- or micro-gravity environments.
A further difficulty is that gas-liquid two-phase systems are unreliable in providing a noble gas to an ion propulsion thruster, which is exacerbated by the sensitivity of ion propulsion systems to propellant flow variations. For example, if liquid enters the flow passageway between the storage vessel and the thruster, it may vaporize to create localized pressure and flow rate increases due to the volume expansion of the liquid to a vapor.
The inventors have also taught the advantages of using an adsorbent as a replacement storage media for compressed gas and two-phase storage of noble gases (U.S. patent application Ser. No. 09/159,387). More specifically, the use of an adsorbent as the primary storage media for a noble gas used in ion propulsion systems can provide less-complex noble gas management and control hardware, reduced storage pressures and stored energy, reduced volumes, and, competitive weights and velocity increments compared with compressed gas and two-phase storage. Overall, however, the storage density of noble gas in a compressed gas or two-phase state is usually superior to noble gas stored on an adsorbent.
The current invention retains some of the weight and storage density advantages associated with the use of compressed gas or two-phase storage of the noble gas as the primary storage media, but teaches the use of adsorbents in combination with the compressed gas or two-phase storage ion propulsion system to (a) simplify and reduce the costs and weight of flow and pressure control hardware, (b) minimize the problems associated with two-phase fluid in a thruster system, (c) provide a means to isolate the primary storage vessel from the thruster components, (d) reduce the number of mechanical parts in a flow and pressure control system, (e) provide a simple means to operate multiple thrusters at multiple flow rates and durations, and (f) provide additional in situ noble gas propellant clean up by adsorbing impurities onto the adsorbent or other catalysts.